Method of and means for heat-treating metal in strip form



March 1945- E. MITTELMANN METHOD OF AND MEANS FOR HEAT-TREATING METAL IN STRIP FORM Filed Aug. 30, 1941 Patented Mar. 13, 1945 METHOD OF AND MEANS FOR HEAT-TREAT- ING METAL IN STRIP FORM Eugen Mittelmann, Chicago, 111.

Application August 30, 1941, Serial No. 408,934

6 Claims.

This invention relates to a method of and means for heat treating metal members and more specifically relates to the art of selectively hardening metal in strip form, such as for example, saw blading.

The primary object of the invention is to provide a novel and improved method of and means for hardening a metal strip.

A further object of the invention is to provide a novel and improved method of and means for hardening a predetermined edge zone of a metal strip.

Another object of the invention is to provide a novel and improved method of and means for hardening a predetermined zone of a continuously moving metal strip.

A further object of the invention is to provide a novel and improved method of and means for heating a metal strip.

A further object of the invention is to provide a novel and improved method of and means for heating a metal strip and establishing a temperature gradient therein.

Still a further object of the invention is to provide a novel method of and means for establishing and maintaining a temperature gradient in a moving metal strip.

Still further objects of the invention lie in the provision of novel apparatus for supporting high frequency induction heater coils novel means for smmmmrip being heat treated; novel means for cooling the heater coils of the high frequency induction apparatus; novel means for compensating for radiation losses of the heated zone of the moving strip as same moves to the quenching stream; novel apparatus for increasing the rate of energy transfer into the moving metal strip at any desired place; novel apparatus for preventing short circuiting of turns of the heater coils of the high frequency induction apparatus.

Still further objects lie in the provision of heater coils of various shapes and configurations designed to heat metal strips by induction of high frequency currents therein in an efilcient manner.

With the foregoing and other objects in view, which will appear as the description proceeds, the invention consists of certain novel features of construction, arrangement and combination of parts, procedures, modes and steps of practice, hereinafter fully described, illustrated in the accompanying drawing, and particularly pointed out in the appended claims, it being understood that various changes in form, proportion, size and minor details of structure, and in details of procedure of the method may be made without departing from the spirit or sacrificing any of the advantages of the invention.

For the purpose of facilitating an understanding of my invention, I have illustrated in the accompanying drawing a preferred embodiment of the apparatus thereof from an inspection of which, when considered in connection with the following description, my invention, its mode of construction, assembly, operation and practice, and many of its advantages should be readily understood and appreciated.

Referring to the drawing in which the same characters of reference are employed to indicate the corresponding or similar parts throughout the several figures of the drawing:

Fig. l is a perspective veiw of one form of an edge heating coil which may be employed in connection with the invention.

Fig. 2 is a view similar to that of Fig. 1, but with a diiferently shaped edge heating coil.

Fig. 3 is a plan view of the edge heating coil and blade shown in Fig. 1.

Fig. 4 is a transverse sectional view taken through the coil shown in Figs. 1 and 3.

Fig. 5 is a transverse sectional view taken through the coil shown in Fig. 2.

Fig. 6 is a transverse sectional view taken through a modified form of edge heating coil.

Fig, 7 is a schematic perspective view showing apparatus, embodying the invention, for the continuous heating and hardening of r a moving saw blade.

Fig. 8 is a perspective view of a modification of a portion of the apparatus of Fig. 7.

Fig. 9 is a sectional view taken through a portion of a modified form of the apparatus shown in Fig. '7.

Fig. 10 is a schematic representation of a modified form of apparatus for the continuous hardening of a moving metal strip.

Fig. 11 is a schematic representation of the field picture perpendicular to the edge of the blade when disposed in a coil, as shown in Fig. 4, the blade being depicted with highly exaggerated thickness to show flux linkage.

In the induction heating of a metal member, the metal is disposed in the field of a coil connected with a high frequency generator. Eddy currents are thereby introduced into the member by rapid change of flux therethrough and the resistance of the member to the passage of said currents serves to heat the member.

To obtain highest efliciency, that is, transfer of maximum energy with minimum of losses, an

extremely closecoupling of the metal member to the coil is desired; and yet, with small members, where the ratio of treated volume to the volume of coil is very small, the coupling is extremely loose. While the amount of coupling depends upon the shape and geometrical configurations of the work piece and the heater coil, nevertheless, for any geometrical configuration, the higher the frequency, the closer the coupling between the coil and the member. Hence, in the case of small members the frequency must be raised considerably if high efliciency is to be obtained. Obviously the use of generators capable of producing frequencies up to about 200 kilocycles per second will thereby be uneconomical so far as the heat treatment of small members is concerned.

I prefer using thermionic generators capable of operating at frequencies of from 1 to megacycles per second. In this range of frequencies, sufficient power is obtained without material restrictions of skin efiect, to produce the desired results where the metal member being treated is in strip form, since this form of cross section gives the lowest ratio of high frequency resistance to D. C. resistance.

In the manufacture of saw blades, and especially band saw blades, the ideal hardness condition consists of having a hardened zone of almost martensitic steel, including the teeth, and a toughened or relatively softened zone comprising the remainder or back of the blade. The two zones, the tooth zone and the back zone, must be produced after the blade has been formed. Each zone must, therefore, be subject to highly divergent temperatures at the same time and quenched while the gradient between the zones exists. The blade, as it comes from the forming process is strain hardened and obviously some type of annealing for the back zone is also essential.

Methods utilizing gas flames augmented with raw oxygen to heat the tooth zone prior to quenching have not been entirely satisfactory. Where a continuous blade is to be treated the problem is acute due to the high heat to which the blade must be subjected for a short time in order to assure a reasonable speed of the moving strip. The back of the blade is thereby hardened by heat conducted from the tooth zone. It is thus the practice to draw the back zone, that is, subject it to a second heat treatment to remove the undesirable hardness therefrom. This, of course, will reduce the hardness in the tooth zone.

The use of gas-oxygen flames gives rise to numerous other disadvantages, the more important of which are as follows:

(1) Variations in the chemical and physical composition of the gas gives rise to non-uniformities in the fiame, resulting in uncontrolled temperature, and hence, uncontrolled hardness.

(2) The use of excess oxygen causes the oxidatlon of the metal and the formation of scale.

(3) The external application of heat melts sharp edges, rounds corners, and dulls the blade.

Each of these problems and disadvantages increases in its proportions as the width of the blade decreases. For example, it is difllcult to obtain satisfactory results using the gas flame method in blades whose width is of the order of /4 of an inch or less.

Induction heating methods in which the blade is passed through the center of a coil parallel to its axis result only in a uniform heating of the blade and therefore do not solve the edge hardening problem. The field density is greatest alongside the turns of the coil, and hence. the treated edge should be disposed close to the coil turns. But in order to induce energy into the blade under these conditions, so much power is demanded of the generator that the use is economically prohibitive. Attempts have been made to subject sheet metal pieces to the field produced at the end of a coil, that is, by disposing the piece perpendicular to the coil axis, just outside of the last turn. The results obtained are far from satisfactory, due to the fact that the field density is low, the coupling extremely loose, the only portion of the actual field used is stray flux, and the amount of power input of the generator which would be necessary to obtain any practical degree of heating would be enormous.

In accordance with my invention, I pass the saw blade first through a coil, the axis of which is parallel to, or coincident with, the path of the moving blade and then I pass the blade perpendicular to the axis of an edge heating coil, well within the center thereof, and between two turns. Figs. 1 to 6 show various form of edge heating coils which may be employed. Thus, as shown in Figs. 1 and 4, the plane of the blade I!) is perpendicular to the axis of the edge heating coil l I. In this manner the familiar fiux pattern of two symmetrical loops, one on either side of the axis of the coil, becomes broken into four loops, two pair, symmetrical on either side of the point at which the blade cuts the axis of the coil. This is symbolically shown in Fig. 11. The blade is shown with exaggerated thickness in order to show the flux lines F linking same and concentrating therein at the point where the coil is entered. It will be noted that of ithe useful fiux, that is, the part of the field within the coil 22, portions F link with the metal member ID, and portions F" which do not link with the metal member are substantially parallel to the axis of the coil while being perpendicular to flux F. This phenomenon is present only in my arrangement, since when a member is arranged in the usual position parallel to the axis, all of the useful fiux threads the piece and there is no concentration at F and hence no flux linking the metal member perpendicular to the non-linking fiux within the coil.

With the blade perpendicular to the axis of the coil, practically all of the flux lines F must pass through the blade at F. Thus the flux density is greater, the amount of eddy currents induced is greater and more concentrated, and greater energy can be quickly transferred from the generator into the blade, than by disposing the blade in any other position with respect to the coil. In order to produce a heating of the tooth zone 12, this portion is disposed immediately adjacent the inner periphery of the coil turns, with the back zone l3 protruding out from the coil and not subject to being heated thereby.

In this manner I find that I can establish sumcient gradient in the blade for certain purposes by quenching immediately that the required temperature is obtained in the tooth zone. Inasmuch as the heating of the blade is accomplished while same acts as-a load in the tank circuit of a thermionic high frequency generator, the exact power absorbed by the load and hence the exact temperature at the work piece may be continuously metered, observed and controlled. Suitable apparatus for metering power absorption of the work piece is described in United States Letters Patent No. 2,240,955, for a High frequency wattmeter, issued to me May 6, 1941.

In order for the blade in to be as closely coupled to the coil as possible. I distort the connecting portion 14 of the helix between two adjacent turns l and 16 of the coil ll so that substantial portions of each of the two turns and I6 between which the blade ID is adapted to pass, are parallel to one another and perpendicular to the axis A of the coil, as shown in Fig. 3. By this means the maximum flux will link with the blade.

Since the flux density is greatest immediately adjacent the inner periphery of the coil and it is desirabl to subject the tooth zone I2 to as much linking flux as possible, the blade is therefore passed through the coil in such a manner that it is under the influence of the densest portion of the field as much as possible. A small chord is cut by the blade when the coil is circular in cross section, the depth depending upon various physical factors. By forming the coil with a rectangular cross section, as shown in Fig. 5 at H, the blade may be subject to the great flux density adjacent the inner coil periphery for a considerable length of time. This is true since the flux pattern within the coil will follow the cross sectional configuration of the coil itself. This is a preferred form of coil because of its ease of fabrication and satisfactory results in comparison with results obtainable from the use of other more complicated shapes which may produce heavier field densities. For example, I have found that the flux density can be greatly increased by means of a coil l8 of the shape shown in Fig. 6. A loop I9 is formed, in one side of each turn and bent inwardly constricting the field and causing a very intense flux density between the loop I! and the base of the coil. The physical constants 01' the coil are all substantially the same as a similar coil not having the inwardly projecting loop 19, but the volume has been decreased, and hence the same amount of flux must traverse a restricted space. If the blade is disposed, as shown in the drawing between the loop is and the base of the coil, it is subject to a highly concentrated field and hence will absorb power from the generator at a very high rate. This shape of coil is more difflcult to form than that shown in Fig. 5.

The selective heating of the saw blade without preheating, as by a coil of Figs. 1 to 6, is satisfactory where the blade is large and conduction of heat to the back zone is not a problem, or where the temperature gradient required is not very high. In such cases the strip is moved under a stream of quenching fluid in due course, without either great loss of heat through radiation or destruction of the temperature gradient between the tooth zone and the back zone through thermal conduction and a satisfactory hardness condition obtainet. Where higher heats or higher temperature gradients are necessary, however, as for example in the hardening of high speed band saw, blades or razor blade strip steel, and

where the size of the blade is so small as to *normally cause the back zone to be heated by conduction prior to quenching. selective heating alone will not yield results as satisfactory as will the use of the method presently described.

This method utilizes means for first establishing a considerable temperature gradient in the saw blade and then raising the temperature of the blade while at the same time maintaining or raising the gradient. For establishing the temperature gradient, the moving blade [0 is led through a coil 20 parallel to the coil axis. The back zone l3 of the blade is entirely shielded by means of a U-shaped guide or shield 2| which is formed of some highly conductive material, such as copper. The exposed portion of the blade, namely the tooth zone I2 is free to be linked by a good deal of a fiux established by the coil, while the remainder of the blade must share the flux with the copper surrounding it, shielding it, and preventing flux from threading it. Such currents as tend to be induced will be induced primarily in the copper shield and such heat as will tend to be produced in the back portion l3 of the blade will be carried away by conduction to the copper shield or guide 2|. It is preferred that the shield be contacted by the blade throughout its length. While a space is shown between the blade and shielding in Fig. 9, it is merely for clarity of the drawing.

The rate of energy transferred to the blade in the preheat coil 20 is somewhat limited due to the stray field effects inherent in arrangements of this type. The coupling is loose due to the small volume of work piece compared to the volume of the coil, and hence the temperatures obtainable are comparatively low if the blade I0 is intended to travel at speeds where commercial blades can be produced economically. However, the temperature gradient produced is the im portant reason for utilizing the preheat coil 20. I find that by the use of such a coil I can preheat a blade of the order of /4 of an inch in width and travelling at a speed of the order of 10 feet per minute to a gradient of 1500" F. in the tooth zone to approximately 600 F. in the back zone.

Following the preheating of the blade; it is passed through a coil 22 arranged with its axis perpendicular to the axis of the coil 20 and hence perpendicular to the blade 10. As explained previously, this is the most efficient type of association for maximum energy transfer rate. The blade I0 is arranged so that the tooth zone I2 is immediately adjacent the inner surface of the upper side of the coil 22. The preheat coil 20 may be circular or rectangular in cross section but it is preferred that the heater coil 22 be rectangular. Instead of permitting the back zone of the blade to protrude outwardy from the coil, as shown in Fig. 5, same will extend inwardly of the coil as shown in Fig. '7 and be shielded to prevent heating thereof. I find this arrangement most practical for the construction of simple apparatus.

The back zone l3 of the blade in is shielded at the coil 22 by a copper shield 23 of which the U- shaped guide 2| is an extension. Thus the only portion exposed to the flux is the tooth zone. The tooth zone is thus heated intensely at the coil 22 so that by the time the coil has been passed by the blade ID. the temperature gradient is, for the same conditions set forth above, from about 1900 F. to about 2400 F. in the tooth zone to about 800 F. to 1000 F. in the back zone. Quenching at this temperature gradient will result in a highly satisfactory blade.

The shield 23 is the upper portion of a member 24 which provides a large volume of thermally and electrically conductive material. The member 24 is mounted on a block 25 of insulating material and suitably attached thereto by screws or the like. The top of the block 25 has a recess 26 within which the bottom of the coil 22 rests and a narrow connecting recess 21 within which the bottom of the preheat coil 20 is disposed. These recesses are adapted to be flooded with quenching fluid, as will presently be described. The leading end of the block 25, as determined by the direction of the travel of the blade l0, (indicated by arrows D), is slotted for the reception of the solid portion 28 of the member 24. This portion extends downwardly from the shielding portion 23, stopping short of the shielding portion 2| in its longitudinal dimension. The bottom of the portion 26 terminates in two large horizontally arranged fins 29 which are adapted to be disposed below the block 25 either attached to the bottom or freely suspended therefrom. It may be convenient to provide openings in the fins 29 for the accommodation of means for fastening the member 24 to the block 25.

The member 24, besides serving as electrical shielding, as hereinbefore described, conducts a considerable portion of the heat radiated by the back zone |3 of the blade Hi to the portions 2| and 23 from said portions 2| and 23 to the cooling portion 28 and the fins 29. Since the portion 28 is in the center of the recess 26 it is constantly being cooled by the quenching fluid with which the recesses 26 and 21 are flooded. The combination of these expedients serves to maintain as great a temperature gradient between the back zone l3 and the tooth zone |2, as possible.

With the gradient of the desired temperature established in the tooth zone, the blade I is passed through a stream of quenching fluid, such as oil, which is caught, cooled and recirculated by appropriate means not pertinent to this invention. A discharge tube for the fluid is designated in the drawing by the reference character 30. The quenching of the heated blade hardens the tooth zone. The heating of the back zone is suflicient to toughen same, regardless of quenching. The blade is converted to these conditions from its original strain hardened state.

Under certain operating conditions, the amount of magnetic material in the field of the coil 22 is insufficient for power absorption of sumcient degree. The reason for this is that the flux concentration, and hence the rate of energy transfer depends upon the actual permeability of the material in the fleld. In shielding the back zone l2, it is desirable not to cover so much of the blade as to effectively remove permeable material from the field and prevent the establishment of optimum transfer conditions. With the establishment of a temperature gradient in accordance with my invention, it has been found plausible and highly advantageous to provide a recess 35 in the shielding portion 23 of the member 24. This uncovers more of the blade and thus places more permeable material in the fleld Of the coil 22 to thereby increase the rate of energy transfer.

In practice the bottom of the groove formed in portions 2| and 23 is provided with a removable block 3|, preferably of copper, to limit downward movement of the blade I0 within the shielding and to thereby define the exposed portion of the blade. Various thicknesses of the member 3| may be made available for various operating conditions.

The entire apparatus including the block 26, member 24, and the coils 20 and 22, may be arranged in any well known manner to be movable in a vertical direction to adjust same with respect to the blade, which may be passing between two suspension points entirely free from the apparatus. Various other means for adjustment of the coils, etc. should suggest themselves to those skilled in the art.

In order to prevent short circuiting of the coils by the shielding members 2| and 23, I utilize mica sheets 32 on the outside of the shielding portions 2| and 23 for insulating purposes, as shown in Fig. 9. For the sake of clarity these sheets have been omitted from Figs. 7 and 8.

While I have shown and described the use of only two coils 20 and 22, I do not wish to limit myself to such restricted use. Various combinations of coils, arranged both parallel to the blade and perpendicular thereto, have been utilized with varying degrees of success and for various purposes. The connections, although shown as series in the drawing, have been made in series, in parallel, and in combinations of series and parallel. It has been found that satisfactory results are obtainable regardless of the type of connection.

I have shown in Fig. 10 a highly satisfactory arrangement in which I provide a preheat coil 20 arranged parallel to the blade, a heater coil 22 arranged perpendicular to the blade, and a radiation loss compensation coil 33 all in series with the generator 34. The radiation loss compensation coil 33 consists of one or so turns of wire arranged adjacent the tooth zone of the blade and intended to keep the tooth zone at a desired temperature from the time it leaves the heater coil 22 to the time it is carried under the quenching tube 30. This coil may consist of a self-supporting loop, either attached to the block 25 or to the coil 22.

In the conventional type of high frequency induction equipment, and especially in cases where high energy transfer rates are required, the heater coils are usually liquid cooled from within the tubes from which they are formed. This procedure is impractical with the equipment which I have described, since the work piece is quite small in cross sectional area and the coils are correspondingly proportioned. For example, I have been utilizing copper wire 1 of an inch in diameter in my coils. A highly efficient cooling arrangement is obtained by resting the bottoms of the coils in the recesses 26 and 21. The recesses are then flooded with quenching fluid which is constantly being circulated. One system utilizes bent surfaces 36 to by-pass a portion of the stream of fluid discharged upon the blade from the tube 30. Other methods may be used for flooding the recesses. In this manner, the quenching fluid serves to cool the blade I, the coils 20 and 22, and the member 24. The heat generated in the coils will obviously be carried by conduction to the bottoms thereof where the temperature is the lowest.

From the above it will be seen that I have devised advantageous methods of and apparatus for establishing a temperature gradient in a moving strip of metal preparatory to qu. nching same for producing diflerent hardness zones therein. Many other arrangements, besides those shown above, could be devised, utilizing my invention. For example, in the methods illustrated by Figs. 7 and 8, the shields in either the pre-heat coil 20 or the heater coil 22 could be omitted and satisfactory results nevertheless obtained.

While I have illustrated my invention in connection with saw blades, it is equally applicable to any type of blading, such as knife blading, and especially razor blading. Double edged blades, for example, could be easily heat-treated by means of my invention by the mere duplication of apparatus to treat both cutting edges as hereinbefore described. Such minor changes are well within the scope of my invention and intended to be covered by me.

It is believed that my invention, its mode of construction and assembly, and many of its advantages should be readily understood from the foregoing without further description, and it should also be manifest that while a preferred embodiment of the invention has been shown and described for illustrative purposes, the structural details are nevertheless capable of wide variations within the purview of my invention as defined in the appended claims.

What I claim and desire to secure by Letters Patent of the United States is:

1. In a high frequency induction heating apparatus, a support comprising a block having connected recesses, a plurality of induction heater coils positioned within said recesses of said block, means for moving a strip of metal to be heated through said heater coils, means for supporting said strip of metal as it is moved by said moving means through said coils, mean for quenching with cooling fluid the metal strip after the strip has been heated by said coil, and means for causing a portion of said cooling fluid to pass through said recesses for contacting and cooling said coils by conduction.

"2. The method of heating a strip of metal which comprises generating first and second high frequency magnetic induction fields in spaced relation with the main flux lines extending substantially perpendicular to each other, longitudinally moving the strip in succession through the first field substantially coincident with the main fiux lines of said field to uniformly preheat the entire strip and then through the second field substantially perpendicular to the main flux lines of said second field and in such manner that the edge of the strip passes through the area of greatest concentration of said main fiux lines to raise the edge only of the uniformly preheated strip to a higher degree.

3. In a high frequency induction heating apparatus for the heating of a moving strip of metal, means for moving said strip, air core heater coils through which said strip is moved in succession by said means, the air core coil through which the strip is first moved having its axis substantially coincident with the longitudinal axis of the moving metal strip to uniformly preheat the entire strip, and the air core coil through which the strip is subsequently moved having its axis perpendicular to the longitudinal axis 01' the moving metal strip and being arranged relative to the strip to heat a localized portion of the preheated strip to a higher degree.

4. In a high frequency induction heating apparatu for the heating of a strip of metal, means for moving said strip longitudinally, air core heater coils spaced along the path of movement of said strip and constructed to permit passage therethrough of said strip as it is moved by said means, one of said coils being positioned With its axis substantially coincident with the longitudinal axis of the metal strip to preheat the strip substantially uniformly as it is passed through said coil, and a second coil to which the strip is moved by said moving means after passing through said one coil, said second coil having its axis substantially perpendicular to the plane of the metal strip as it is moved through the second coil by said moving means and arranged to provide a maximum density of flux linkages in a desired localized portion of the strip to supply additional heat to said localized portion of the uniformly preheated strip.

5. In a high frequency induction heating apparatus, a coil substantially rectangular in cross section and comprising relatively helical convolutions with a portion connecting two adjacent convolutions extending transversely of the plane of the path of the moving strip but out of said path, said adjacent convolutions being displaced from a true helix into planes substantially perpendicular to the axis of the coil and suificiently spaced apart axially oi the coil to permit the moving strip of metal to pass therebetween, and one of said adjacent convolution having a portion offset radially toward the axis of the coil.

6. In a high frequency induction apparatus for heating a strip of metal, a pair of non-magnetic strips, mean for moving said metal strip longitudinally between said non-magnetic strips, an air core coil surrounding said non-magnetic strips and the strip of metal as it is moved between said non-magnetic strips with the axis of said coil extending substantially parallel to said non-magnetic strips for uniformly heating said metal strip as it is moved between said non-magnetic strips, and a second air core coil having sections on opposite sides of said non-magnetic strips with its axis substantially perpendicular to said non-magnetic strips and the metal strip as it is moved between said non-magnetic strips, said non-magnetic strips being constructed to expose a desired portion of the metal strip to the magnetic flux lines of said second air core coil while ,shielding w the remaining portion of the strip from said magnetic flux lines 01' said second air core coil whereby to effect a localized heating of the strip by said second coil.

EUGEN MI'I'IELMANN. 

